Aircraft may, without ATC clearance, fly at right angles across the base of an en-route section of an Airway where the lower limit is defined as a Flight Level.
Aircraft may ALSO, without ATC clearance, fly at the base of ANY controlled airspace in ANY direction when the lower limit is defined in ANY way [except, of course, GND].
By ICAO definition, the boundary between airspace of two different classes is the lower of the two classes, so the base level of an airway above the open FIR is Class G.
Although doing so is probably bad airmanship considering climbing by 1 ft would result in an infringement.